Tennis racquets, squash racquets, racquetball racquets, and badminton racquets have a head portion, supporting a string bed, and a handle. Typically, the string bed is formed by a plurality of main strings, which extend generally parallel to the longitudinal axis of the racquet, and cross-strings which extend perpendicular to the racquet axis and which are interwoven with the main strings. Alternatively, in some racquets, the two sets of interwoven strings extend diagonally.
Although it is common to refer to a racquet as having a plurality of main strings and cross-strings, the racquet is actually strung with one or two lengths of string. A plurality of string holes are bored through the frame, and grommet pegs, formed on a grommet strip or bumper strip, are inserted through the holes to act as guides for, and to protect, the string. The string is threaded so as to extend between successive pairs of string holes lying on opposite sides of the frame. After the string crosses the string bed and exits one string hole, it runs along the outside surface of the frame, enters the next string hole, and then crosses the string bed in the opposite direction.
The strings need to be tensioned at a predetermined level, which is normally within a range set by the manufacturer for the particular racquet model. A racquet stringing machine is used for this purpose. A simplified drawing of a prior art racquet stringing machine is shown in FIG. 1. The machine includes a base 10, which is rotatably supported on a post 12. A pair of risers 14, 16 are supported on the base 10 so that they can be moved toward and away from one another in order to secure a racquet frame. Typically, the risers 14, 16 engage the inside surface of the frame at the tip (12 o'clock position) and throat (six o'clock position).
The machine includes a tensioning device 18, shown only schematically, which is either fixed or moveable relative to the post 12, and one or more string clamps 20. The lower end of each clamp 20 is slidable along a shaft 22. The ends of the shaft 22, in turn, are supported in guide blocks 24 for movement along a pair of opposed guides 26 (one of which is shown in FIG. 1), that extend perpendicular to the shaft 22. In this manner, the clamp 20 can be moved in any direction parallel to the plane of the base 10.
The string clamp 20 is used for stringing the cross-strings. A similar string clamp, supported on a similar pair of guide blocks, may be mounted to move along a second pair of guides 28, oriented perpendicular to the guides 26, for stringing the main strings.
In order to string a racquet, the racquet frame is first secured to the stringing machine. A length of string is then laced through the center main string holes. The amount of string laced through the holes depends on whether the racquet is to be strung with one or two lengths of string. If the racquet is be to strung with two lengths of string, then the amount of string laced through each pair of holes is equally divided. If the racquet is to be strung with one length of string, then the length of string needed to string one-half of the main strings is laced through one of the holes. The remainder of the string, enough to string the other half of the main strings plus all of the cross strings, is laced through the other pair of holes.
Stringing begins by securing one of the loose strings relative to the frame using a string clamp. The unclamped string is then tensioned by the tensioning device 18 and clamped to hold the tension. Each string segment is tensioned and clamped, one at a time, starting with the center main strings and working outwardly. In tensioning successive string segments, the base 10 can be rotated 180.degree. in order to allow the tensioning device 18 to pull successive string segments alternately in opposite directions. The tensioning and clamping of the main strings preferably alternates from side-to-side so that the stress on the frame while installing the main strings is uniformly spread across the racquet face. After all the main strings are installed, one or both ends of the string are tied off depending on whether it is a one or two piece string job. The cross strings are then installed from either top to bottom, or bottom to top, depending on the manufacturer's recommendations. The first cross string is woven between the existing main strings and then tensioned and clamped to retain the tension. Each succeeding cross string is alternately woven, that is, woven over main strings that the preceding cross string was woven under and so forth. This procedure is continued until all cross strings are installed in order to form a uniform mesh.
When stringing the racquet, it is necessary to hold the racquet frame securely on the stringing machine. Older machines (i.e., pre-dating FIG. 1) had a very simple mounting system, in which a pair of metal posts were secured to the risers 14, 16. The racquet frame was positioned so that the metal posts were located inside of the frame head, at the 6 o'clock and 12 o'clock positions, respectively. The risers were then moved apart until the metal posts contacted the opposed inside surfaces of the frame, and the racquet was clamped down on the riser.
Typically, the main strings are strung first, so that the pulling force of the partially strung racquet, which would tend to cause the frame to deform (i.e., because there is no cross-string force yet to oppose it), could be born by the metal posts. Using metal posts to bear all of the force of the main strings was not a problem in older racquets, because most were made of solid wood, the inner hoop was flat (producing a relatively large contact area between the frame and the posts), and string tensions were relatively low.
Commonly owned U.S. Pat. No. 3,999,756 discloses a tennis racquet in which the geometric proportions of the conventional racquet were changed to provide a larger head and longer strings. Virtually all tennis racquets made today utilize such geometry. Commonly owned U.S. patent application Ser. No. 08/279,837 discloses a squash racquet geometry, widely used today, in which the throat bridge is eliminated and the strings extend down into the throat area, resulting in longer main strings. More recently, commonly owned U.S. Pat. Nos. 4,531,738 and 4,618,148 disclose racquetball racquets in which the geometry of the conventional racquet is changed to produce a larger head size, again resulting in longer strings. Most racquetball racquets today use this large head geometry.
As the head sizes of racquets increased, and the strings have become longer, the desired stringing tensions have increased. Also, today most sports racquets are made with composite materials, rather than wood or metal. Because such frames can be molded into any desired cross-sectional shape, the inside wall of the frame is usually curved rather than flat. As a result, the metal post of a conventional stringing machine tends to make point contact with the inside frame surface, creating a region of high stress. This problem is compounded by the fact that most composite racquet frames are hollow, and the contact point happens to be at the mid-span of the racquet contour, i.e., where the frame is weakest, especially with the higher cross-sections of today's widebody frames.
On typical modem stringing machines, such as those sold by the assignee under the trademarks Prince.RTM. and Ektelon.RTM., the support system, referring again to FIG. 1, has a pair of retainers 30, 32, mounted on the tip riser 16 and a throat riser 14, respectively, and hold down clamps 34, 36, in place of the posts of the older stringing machines. A stringing machine of this type is also disclosed in commonly owned U.S. Pat. No. 4,417,729.
In order to secure the racquet frame properly, it is important to use retainers 30, 32 that have the proper geometry for the particular racquet model being strung. Known retainers have various shapes, such as an are, an inverse are, or V-shape. The Prince.RTM. stringing machine currently has around 15 different retainers, and the machine is designed to accept any of these interchangeable retainers.
While these custom retainers ensure that the racquet frame is clamped securely without being damaged, and while the ability to change retainers allows the machine to be used with virtually any racquet, it is inconvenient to have to stock so many different retainers, and there is the potential problem that some of the retainers will get lost or mislaid.
In addition to a mechanism to adjust the distance between the risers 14, 16, some machines permit the relative heights of the risers 14, 16 to be adjusted. The purpose of adjusting the relative heights of the risers is to level the string bed in racquets where the frame heights at the tip and throat midspan are different. Absent a way to adjust the relative heights of the risers, the string bed in such a racquet would not be level. It is desirable to keep the string bed level when stringing the racquet because the string clamps work best if the string bed is level, and because the frictional forces between the string and the grommets will be more uniform, allowing the strings to be tensioned more uniformly and accurately.
In order to mount a racquet, after selecting and installing the proper retainers, the distance between the risers is adjusted to leave a little space between the retainer and the racquet. The height of the risers is then adjusted (if such feature is available on the machine) to compensate for any height differences between the tip and throat.
The tip of the racquet is then secured to the tip riser 16, by positioning the frame between the retainers 30 and tightening the clamp 34. Once this is done, the final adjustment of the distance between the risers is made, so that the throat will be properly positioned relative to the throat retainers 32 when the throat retainer clamp 36 is tightened. The throat is then secured to riser 14 by tightening clamp 36. Such procedure can require substantial time in terms of adjusting and sometimes readjusting before actual stringing can begin.
A modem version of the old type of stringing machine, which includes upright posts rather than retainers for securing the frame, is sold under the name Babolat 3000. The metal posts are covered by a piece of molded nylon which is radiused to protect the racquet's inner hoop. The machine also has two pairs of arms, or outriggers, with V-shaped blocks mounted on the ends. These outriggers are positioned on the outside of the racquet frame so that the V-shaped blocks contact the outside surface of the racquet frame at the 10 o'clock, 2 o'clock, 4 o'clock, and 8 o'clock positions. This mechanism is described further in U.S. Pat. No. 5,026,055.
A racquet is mounted on the Babolat machine by opening the outriggers, and reducing the distance between the metal retaining posts until the frame can be positioned between the outriggers and the retaining posts can fit inside the tip and throat areas. The racquet is then laid on the risers, and both sets of outriggers are adjusted inwardly until they make contact with and raise the racquet off the risers, centering it in the V-blocks. At this point, the retaining posts are moved away from one another until they make firm contact at the 12 o'clock and 6 o'clock positions of the racquet head.
As discussed above, when the racquet is partially strung, the main strings will pull the tip and heel regions toward one another, which force is resisted by the retainers or posts at the tip and throat regions. The Prince and Ektelon stringing machines support the racquet frame only at the 6:00 and 12:00 positions, in order to allow the racquet to breathe naturally and without restriction. In the case of the Babolat machine, the purpose of the outriggers is to attempt to keep the racquet head from changing shape during the time the racquet is only partially strung.
While the outriggers act to self-level the string bed, a Babolat-type machine has certain drawbacks. The type of two-point mounting in the Babolat-type machine, at the 6 o'clock and 12 o'clock positions, is undesirable because a two-point loading alone cannot stabilize the racquet. Thus, outriggers are required to prevent twisting and side-to-side movement. However, outriggers are rather bulky, tend to get in the way when stringing the racquet, and increase the amount of time needed to mount the racquet on the machine. Also, it would be difficult to string a racquet with an odd number of main strings, because the center main string would extend along the centerline of the head, where the two retaining posts are located.